The goal of plant breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits may include greater yield, resistance to diseases, insects or other pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, enhanced growth rate and improved fruit properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same genotype. A plant cross-pollinates if pollen comes to it from a flower of a different genotype.
Plants that have been self-pollinated and selected for (uniform) type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny of homozygous plants. A cross between two such homozygous plants of different lines produces a uniform population of hybrid plants that are heterozygous for many gene loci. The extent of heterozygosity in the hybrid is a function of the genetic distance between the parents. Conversely, a cross of two plants each heterozygous at a number of loci produces a segregating population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties requires the development of homozygous inbred plants, the crossing of these inbred plants to make hybrids, and the evaluation of the hybrids resulting from the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selfing and selection of desired phenotypes. The new plants are evaluated to determine which have commercial potential.
Cucumber (Cucumis sativus L.) is naturally a diploid (2n=14) outcrossing species, although haploid, doubled-haploid (see, e.g., U.S. Pat. No. 5,492,827 hereby incorporated by reference in its entirety), and triploid (see, e.g., Sarreb et al. (2002), Plant Cell Tissue, Organ Culture 71: 231-235) types have been developed. The two main types of cucumber fruit grown commercially today in the United States are fresh market (slicing) type and the processing (pickling) type. Varieties and production methods are typically adapted to the end use. Slicing cucumbers are often longer, larger and have darker and thicker skin, whereas pickling/processing cucumbers have a shorter fruit, thinner skin with interior flesh that make them more amenable to pickling. Seedless varieties are generally preferable for both fresh market and for pickling as seeds are not palatable.
Cucumber plants that set fruit parthenocarpically (without pollination and fertilization) have more recently been available. These plants produce seedless fruit unless pollinated. Growth of parthenocarpic varieties is beneficial in that setting of fruit on these cultivars does not produce an inhibiting effect on plant growth, unlike the case of fertilized, seeded fruit. The seedless varieties are usually higher yielding and of higher quality due to the lack of seeds. However, growth of these plants requires isolation from seeded cucumbers to avoid pollination and subsequent seeded fruit.
Most of the cucumbers currently grown for processing to pickles and pickle products in the United States are seeded hybrid varieties. Hybrid varieties offer the advantages of easy combination of dominant and recessive traits, such as disease resistance, from a set of inbred parents, as well as careful control of parentage. The production of F1 hybrid cucumber seeds from a pollen parent bearing only male flowers has been reported (see, e.g., U.S. Pat. No. 4,822,949).
Many different cucumber cultivars have been produced, and cucumber breeding efforts have been underway in many parts of the world (see e.g., U.S. Pat. No. 6,765,130). Some breeding objectives include varying the color, texture and flavor of the fruit. Minimizing the occurrence of bitterness in cucumbers is one such example. Other objectives include optimizing flesh thickness, solid content (% dry matter), and sugar content. Also, breeding programs have focused on developing plants with earlier fruit maturity; more restricted vine growth, improved disease resistance or tolerance, and improved adaptability to environmental conditions.
Advances in biotechnology have also resulted in genetically engineered cucumber plants with improved traits. For example, cucumbers resistant to CMV have been developed by expression of CMV protein coat genes (see e.g., U.S. Pat. No. 5,349,128). Transgenic plants exhibiting, for example, other viral resistance traits or high levels of superoxide dismutase have also been reported (see e.g., U.S. Pat. No. 6,084,152).
While breeding efforts to date have provided a number of useful cucumber varieties with beneficial traits, there remains a great need in the art for new varieties with further improved traits. Such plants would benefit farmers and consumers alike by improving crop yields and/or quality. Some breeding objectives include varying the color, texture and flavor of the fruit, and absence of seeds. Other objectives include disease or pest resistance, optimizing flesh thickness, yield, suitability to various climatic circumstances, solid content (% dry matter), and storage properties.